1. Field of the Invention
The present invention relates to a vehicle automatic transmission, and more particularly to a control of a vehicle automatic transmission having a torque converter and a multiple stage transmission gear mechanism connected with the torque converter.
2. Description of the Prior Art
Conventional vehicle automatic transmissions having a torque converter anda multiple stage transmission gear mechanism connected to the torque converter include hydraulically actuated friction members for selecting one of the gear stages of the transmission gear mechanism in accordance with vehicle operating conditions. An example of such automatic transmission is shown in the Japanese patent application No. 55-61070 filed on May 7, 1980 and published for public inspection on Dec. 3, 1981 under the diclosure No. 56-156543. In this automatic transmission, two friction members are oppositely actuated to select two adjacent gear stages. For example, under an operation in D-range, the third gear stage is obtained by having one of the clutches engaged and one of the brakes released but the second gear stage is obtained by having the one clutch disengaged and the one brake engaged. In order to operate the clutch and the brake in the opposite direction by the hydraulic pressure supplied through a common fluid line, the clutch is of a type wherein it is normally engaged but disengaged under a hydraulic pressure whereas the brake is of a type wherein it is normally released but engaged under the hydraulic pressure.
In an automatic transmission having the aforementioned control system, it is essential to determine the timings of the operations of the friction members because otherwise there will be produced uncomfortable gear shift shock. Speaking more specifically, when the transmission gear mechanism is shifted from a higher gear stage down to a lower gear stage, there will be an increase in the speed of the torque converter turbine by a value corresponding to the difference between the gear ratios of the higher gear stage and the lower gear stage as soon as the clutch is released. In order to accomplish a smooth shifting down, it is required to complete engagement of the brake when the torque converter turbine speed is increased by the value corresponding to the difference between the gear ratios of the higher and lower gear stages. It should however be noted that it is very difficult to control the operations of the clutch and the brake in a desirable manner throughout the vehicle operating conditions because the rate of the change of the turbine speed varies depending on the various vehicle operating conditions such as the engine speed. Thus, sometimes the vehicle speed, and the engine speed. Thus, sometimes the clutch may be engaged before the brake is sufficiently released producing a double-lock phenomenon by which the torque converter turbine speed is decreased than the value suitable for the higher gear stage. Conversely, if the timing of the engagement of the clutch is later than the ideal timing, the turbine speed will be increased beyond the value suitable for the lower gear stage producing a raising phenomenon.
The aforementioned Japanese patent publication proposes to solve the problem by providing a flow restriction in a passage for draining the line to the actuator for the brake so that the brake releasing pressure is relieved in a controlled manner. The proposed solution is considered as being effective in controlling the timings of the clutch and the brake in a desirable way. It should however be noted that even with this proposal, it would not be possible to obtain desired operating timings of the clutch and the brake. A further problem in this proposed arrangement is that the drain line is used in common for the clutch and the brake so that there will be produced changes in time period wherein the pressure is released and the shift shock will not be satisfactorily suppressed.